The present invention relates to a synchronous frequency synthesis method for a radio transmitter-receiver utilizing a phase locked loop.
Regarding the frequency synthesizer utilizing a phase locked loop, there have been proposed several methods which use voltage-controlled oscillation frequencies directly as transmitting and receiving carrier frequencies. In an example where a prescaler is installed to perform frequency division so as to shift up the receiving frequency to obtain the transmitting frequency and where the voltage-controlled frequency is changed at every transmit-receive switching, the self-oscillation frequency of the voltage-controlled oscillator is the same as the transmitting carrier frequency at the amplitude modulation stage and therefore side bands created in amplitude modulation tend to stray into the voltage-controlled oscillator thereby to cause frequency modulation; for this reason, the oscillator section is enclosed with a secure and expensive shield. In this example, the voltage-controlled oscillation frequency is excessively high, and therefore the phase locked loop (PLL) control system is low in stability and in addition, circuit parts must satisfy severe requirements because of a wide range of the voltage-controlled oscillation frequency to be controlled when switching is performed from reception to transmission. In another example where many mixers are used inside and outside of the PLL system such as in the method which uses the oscillation frequency of a single crystal-oscillator as an off-set frequency to perform a digital shift equivalent to the second intermediate frequency when reception is switched to transmission, deterioration of spurious properties are inevitably incurred and, as a result, separate circuit elements or additional parts are disadvantageously required for improvement of spurious properties. Furthermore, the essential part of the above-mentioned PLL is in the form of an integrated circuit of the ECL, TTL or Schottky type, which is inherently large in power consumption and becomes increasingly larger in size if it is to control channel display and peripheral conditions such as digital logic functions, in addition to basic control items.